Methods and apparatus for producing transmission failure protected, bridged, and dispersion resistant signals

ABSTRACT

Apparatus for creating a communication signal, comprising a modulator adapted to: modulate a first and a second beam of continuous wave electromagnetic radiation with a source signal, assemble modulated portions of the first and second beams into a first electromagnetic radiation signal of interposed regular and alternate data bit sequences comprising asserted non return to zero coded data bits, each of the data bit sequences being interposed by unasserted data bits, in which mutually adjacent asserted data bits are conjoined, and assemble modulated portions of the first and second beams into a second electromagnetic radiation signal of interposed regular and alternate data bar bit sequences comprising asserted non return to zero coded data bar bits representing the unasserted data bits, each of the data bar bit sequences being interposed by unasserted data bar bits representing the asserted data bits, in which mutually adjacent asserted data bar bits are conjoined. Such apparatus for creating a communication signal, further comprising means for modulating the first electromagnetic radiation signal with the source signal to shift the phase of the alternate data bit sequences; and means for modulating the second electromagnetic radiation signal with the source signal to shift the phase of the alternate data bar bit sequences. Methods for creating communication signals.

FIELD OF THE INVENTION

The present invention relates to advantageous methods and apparatus forproviding electromagnetic radiation signals that are protected against asignal transmission failure, that are bridged, and that are resistant tochromatic dispersion.

BACKGROUND OF THE INVENTION

Optical telecommunication systems require that a given signal beefficiently transmitted from an origination point to an intendedtermination point, while maintaining a signal quality that is suitablefor use of the signal upon its delivery. The ever present drive forlower costs, higher bandwidth, and faster service creates challenges tothe maintenance of acceptable signal quality. Meanwhile, signal qualitydemands also simultaneously evolve, creating increased systemperformance requirements that often are in conflict with the drive forlower costs.

One of the many prevalent signal quality issues concerns signal failureprotection. A signal transmitted over a particular path is vulnerable tofailure of the equipment supporting the path. To address thisvulnerability, a copy of the signal can be provided for transmission bya different path. Although failure protection by providing such signalcopies is broadly desirable, there are costs associated withimplementation of such protection. For example, splitting of theoriginal signal into two separate signals typically requires either theuse of additional resources to generate and provide energy to carry thesecond signal, or acceptance of the attenuation resulting from directsignal splitting.

Another signal quality issue concerns chromatic dispersion. A typicalsignal is transmitted within a narrow band of wavelengths brackets adesired center wavelength, which wavelengths nevertheless travel atslightly different speeds. On an extended signal transmission path,these different speeds cause a given signal pulse to spread out in time.This pulse spreading, also referred to as chromatic dispersion, canresult in partial overlapping of adjacent pulses within the signal.Herein, a bit signified by the presence of pulse energy is said to be“asserted,” whereas a bit signified by the absence of pulse energy issaid to be “unasserted.” Although an asserted bit is often interpretedas having the value “one” and an unasserted bit as having the value“zero,” other interpretations are also possible. If constructiveinterference occurs in the overlap region between adjacent pulses, aportion of the original signal intended to register as a zero orunasserted data bit, can be distorted to incorrectly register as a oneor an asserted data bit.

There is thus a need for apparatus capable of generating improvedtelecommunication signals that are provided with failure protection aswell as resistance to chromatic dispersion, and a need for methodscapable of generating, transmitting and receiving such signals.

In particular, such apparatus and methods are needed in the context ofnon return to zero (NRZ) coding, in which mutually adjacent asserteddata bits are “conjoined;” that is, the pulse energy does not return tozero between such bits, but instead, those bits are directly joinedtogether into a unitary asserted data bit sequence having a lengthequivalent to the cumulative data bit lengths that are so joinedtogether.

SUMMARY OF THE INVENTION

In one embodiment according to the present invention, an apparatus forcreating a communication signal is provided, comprising a modulatoradapted to: modulate a first and a second beam of continuous waveelectromagnetic radiation with a source signal, assemble modulatedportions of said first and second beams into a first electromagneticradiation signal of interposed regular and alternate data bit sequencescomprising asserted non return to zero coded data bits, each of saiddata bit sequences being interposed by unasserted data bits, in whichmutually adjacent asserted data bits are conjoined, and assemblemodulated portions of said first and second beams into a secondelectromagnetic radiation signal of interposed regular and alternatedata bar bit sequences comprising asserted non return to zero coded databar bits representing said unasserted data bits, each of said data barbit sequences being interposed by unasserted data bar bits representingsaid asserted data bits, in which mutually adjacent asserted data barbits are conjoined.

In this regard, “regular” and “alternate” data bit sequences and databar bit sequences are distinguished from each other by a relativedifference in phase, as will be explained in detail below.

In another embodiment according to the present invention, such anapparatus for creating a communication signal is provided, furthercomprising: means for modulating said first electromagnetic radiationsignal with said source signal to shift the phase of said alternate databit sequences; and means for modulating said second electromagneticradiation signal with said source signal to shift the phase of saidalternate data bar bit sequences.

In a further embodiment according to the present invention, a method ofcreating a communication signal is provided, comprising the steps ofmodulating a first and a second beam of continuous wave electromagneticradiation with a source signal, generating a first electromagneticradiation signal of interposed regular and alternate data bit sequencescomprising asserted non return to zero coded data bits, each of saiddata bit sequences being interposed by unasserted data bits, in whichmutually adjacent asserted data bits are conjoined, and generating asecond electromagnetic radiation signal of interposed regular andalternate data bar bit sequences comprising asserted non return to zerocoded data bar bits representing said unasserted data bits, each of saiddata bar bit sequences being interposed by unasserted data bar bitsrepresenting said asserted data bits, in which mutually adjacentasserted data bar bits are conjoined.

In an additional embodiment according to the present invention, such amethod of creating a communication signal is provided, comprising thefurther steps of modulating said first electromagnetic radiation signalwith said source signal to shift the phase of said alternate data bitsequences; and modulating said second electromagnetic radiation signalwith said source signal to shift the phase of said alternate data barbit sequences.

A more complete understanding of the present invention, as well asfurther features and advantages of the invention, will be apparent fromthe following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary embodiment of an apparatus in accordance withthe present invention for producing a chromatic dispersion resistantelectromagnetic radiation signal protected against a signal transmissionfailure;

FIG. 2 shows a series of sample pulses from a source signal, to beprocessed by the apparatus shown in FIG. 1 in accordance with thepresent invention;

FIG. 3 shows encoded representations of a modified source signal whichcan be produced during initial processing by the apparatus shown in FIG.1 in accordance with the present invention;

FIG. 4 shows encoded representations of a modified source signal duringsubsequent processing by the apparatus shown in FIG. 1 in accordancewith the present invention;

FIG. 5 shows an encoded chromatic dispersion resistant signal, protectedagainst a signal transmission failure, having been processed by theapparatus shown in FIG. 1 in accordance with the present invention;

FIGS. 6A and 6B show an exemplary method according to the presentinvention that can be executed by the apparatus shown in FIG. 1, forproducing failure protected alternate block phase inversion codedrepresentations of a source signal;

FIG. 7 shows an additional exemplary embodiment of an apparatus forproducing a chromatic dispersion resistant electromagnetic radiationsignal protected against a signal transmission failure in accordancewith the present invention;

FIG. 8 shows a plot of bit error rate versus the optical signal to noiseratio required to transmit a signal according to the present inventionover a distance of 80 kilometers; and

FIG. 9 shows a plot of optical signal to noise ratio versus distancetraveled by a signal according to the present invention.

DETAILED DESCRIPTION

The present invention will now be described more fully with reference tothe accompanying drawings, in which several presently preferredembodiments of the invention are shown. This invention may, however, beembodied in various forms and should not be construed as being limitedto the embodiments set forth herein. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.

FIG. 1 shows an exemplary embodiment of a suitable apparatus 100 thatcan be used to produce a chromatic dispersion resistant electromagneticradiation signal protected against a signal transmission failure inaccordance with the present invention. The apparatus 100 includes atransmitter 102, an optical network 104, and a receiver 106. Theapparatus further includes a continuous wave laser 108, an optical 1×2splitter 110, an optical to electrical converter 112, and an inverter114. The apparatus also includes an external modulator 116, phasemodulators 118 and 120, and a toggle flip flop circuit 122.

The transmitter 102 transmits an optical source signal on waveguide 124to optical to electrical converter 112. Optical to electrical converter112 outputs an electrical signal on conductor 126 which is input intotoggle flip flop circuit 122. Optical to electrical converter 112 alsooutputs an electrical signal on conductor 128 which is input intoexternal modulator 116. Continuous wave laser 108 outputs a continuousoptical wave on waveguide 130 to optical 1×2 splitter 110. Optical 1×2splitter 110 outputs continuous optical waves on waveguides 132 and 134that are input to external modulator 116. External modulator 116 outputsan optical signal, generated by modulation of the continuous wavereceived from waveguide 132, on waveguide 136 to phase modulator 118.External modulator 116 also outputs an optical signal, generated bymodulation of the continuous wave received from waveguide 134, onwaveguide 138 to phase modulator 120. Toggle flip flop circuit 122outputs an electrical signal on conductor 140 which is input to phasemodulators 118 and 120. Phase modulator 118 outputs an optical signal onwaveguide 142 which is input into optical network 104. Phase modulator120 outputs an optical signal on waveguide 144 which is also input intooptical network 104. After passing through the optical network 104, theoptical signal received from waveguide 142 is input on waveguide 146 toreceiver 106. After passing through the optical network 104, the opticalsignal received from waveguide 144 is input on waveguide 148 to inverter114, and then on waveguide 150 to receiver 106.

In operation of the apparatus 100, the transmitter 102 emits an opticalsource signal on waveguide 124 that needs to be transmitted to thereceiver 106. The continuous wave laser 108 generates a continuous waveof electromagnetic radiation which is directed on waveguide 130 intooptical 1×2 splitter 110. The output power of the continuous wave laser108 is adjusted to compensate for the attenuation resulting frominterposition of the optical splitter 110. Two identical continuouswaves of electromagnetic radiation are emitted by optical splitter 110on waveguides 132 and 134.

In one preferred embodiment according to the present invention, each ofthe continuous waves of electromagnetic radiation may have a centerwavelength, for example, within a range between about 1525 nanometers(nm) and about 1610 nm. The optical signals transmitted through theoptical network 104 will have the same center wavelength. However,optical signals having other center wavelengths can alternatively beemployed. Further, electromagnetic radiation having a center wavelengthoutside the wavelength range of optical signals can also be employed.

The continuous waves of electromagnetic radiation on waveguides 132 and134 are directed on separate paths into external modulator 116. Externalmodulator 116 is capable of causing constructive and destructiveinterference between the two identical continuous waves ofelectromagnetic radiation input on waveguides 132 and 134. Externalmodulator 116 further is capable of receiving an external electroniccontrol signal which is then used to modulate the constructive anddestructive interference between the two identical continuous waves ofelectromagnetic radiation input on waveguides 132 and 134. Externalmodulator 116 may comprise, for example, a dual output intensitymodulator. The optical source signal is directed on waveguide 124 intothe optical to electrical converter 112, which converts the sourcesignal from an optical form into an electrical form. The resultingelectrical signal is then emitted from the optical to electricalconverter 112 on conductor 126 and is directed on conductor 128 into theexternal modulator 116.

The electrical signal provided on conductor 128 is then used by theexternal modulator 116 to facilitate and control the constructive anddestructive interference between the two identical continuous waves ofelectromagnetic radiation input on waveguides 132 and 134. Constructiveinterference of the electromagnetic wave input on waveguide 132 with theelectromagnetic wave input on waveguide 134 results in propagation of anactive signal, or asserted data bits, referred to as a data bit sequencesignal on waveguide 136. Destructive interference between such waves ofelectromagnetic radiation results in an absence of propagation ofasserted data bits on waveguide 136, which absence is referred to asunasserted data bits. The electrical signal on conductor 128 is alsoused by the external modulator 116 to control the constructive anddestructive interference of the electromagnetic wave input on waveguide134 with the electromagnetic wave input on waveguide 132, resulting inpropagation of a series of asserted data bits, referred to as a data barbit sequence signal, on waveguide 138 as modulated by the source signalfrom transmitter 102. Mutually adjacent asserted data bits are joinedtogether without return of the signal to zero.

External modulator 116 is adapted so that the phase of the interferenceon waveguide 134 is inverted as compared with the interference appliedon waveguide 132. Accordingly, whenever an asserted data bit isgenerated on waveguide 136, there is no signal generated on waveguide138. Conversely, whenever there is no signal, or unasserted data bits,generated on waveguide 136, an asserted data bit is generated onwaveguide 138. Hence, the signal generated on waveguide 138 is theinverse of the signal generated on waveguide 136, and the signalgenerated on waveguide 138 is accordingly referred to as a data bar bitsequence signal. At any given point in time, a data bit is generated oneither waveguide 136 or on waveguide 138, but never simultaneously onboth waveguides. External modulator 116 thus acts as an optical switch,generating a copy of the source signal on waveguide 136, and generatingan inverted copy of the source signal on waveguide 138. The data bitsequence signal is then directed by waveguide 136 into phase modulator118. The data bar bit sequence signal is then directed by waveguide 138into phase modulator 120.

The electrical signal on conductor 126 is also used to control the phasemodulators 118 and 120. The electrical signal on conductor 126 isdirected into toggle flip flop circuit 122, which senses transitions inthe source signal from asserted data bits to unasserted data bits. Thetoggle flip flop circuit 122 is adapted to emit either a relatively highvoltage or a relatively low voltage. Whenever the toggle flip flopcircuit 122 senses a transition in the source signal from asserted databits to unasserted data bits, the toggle flip flop circuit 122 flipsfrom its most recent voltage state. If such a transition occurs when thetoggle flip flop circuit 122 is emitting high voltage, then toggle flipflop circuit 122 flips to emit low voltage. If such a transition occurswhen the toggle flip flop circuit 122 is emitting low voltage, thentoggle flip flop circuit 122 flips to emit high voltage. The voltagecurrently emitted by toggle flip flop circuit 122 is directed onconductor 140 to control the phase modulator 118 and the phase modulator120. When high voltage is applied to phase modulator 118, data bitsequences within the data bit sequence signal remain in phase with thesource signal. When low voltage is applied to phase modulator 118, databit sequences within the data bit sequence signal are shifted to about180° out of phase with the source signal. This phase shift is equivalentto about one half the center wavelength of the carrier signal. Since thevoltage emitted by toggle flip flop circuit 122 changes with everytransition in the source signal from asserted data bits to unasserteddata bits, alternating data bit sequences within the data bit sequencesignal on waveguide 136 are shifted by phase modulator 118 to about 180°out of phase with each other. As chromatic dispersion over a significantdistance leads to pulse broadening and overlapping of adjacent data bitsequences, they destructively interfere due to this alternate phaseshifting. Hence, so long as the overlap of adjacent data bit sequencesis of a magnitude less than about ⅓ of an asserted data bit, pulsebroadening interference does not result in undue corruption of theoriginal data bit sequence signal. The data bit sequence signal is thusprotected from chromatic dispersion.

At any given point in time, a data bit is generated on either waveguide136 or on waveguide 138, but never simultaneously on both waveguides.Hence, asserted data bit signals on waveguides 136 and 138 also aremutually separated by a time period equal to one data bit. For example,a data bit sequence of “101010” on waveguide 136 will be reflected as aninverted data bar bit sequence of “010101” on waveguide 138.Accordingly, the electrical signal on conductor 140 can also be used tocontrol phase modulator 120, provided that this one data bitdifferential is taken into account. In one embodiment according to thepresent invention, the voltage transitions in the toggle flip flopcircuit 122 are applied to phase modulator 118 with a time advancementconstituting about half of one data bit length, so that voltage changesare invoked before passage of the rising edge of the triggering data bitsequence. The time period occupied by transmission of one asserted databit may be, for example, within a range between about 50 and 150picoseconds, such as, for example, about 100 picoseconds. Accordingly,for example, the voltage transitions can be advanced by a time periodwithin a range between about 25 picoseconds and about 75 picoseconds,such as, for example, a time period of about 50 picoseconds. The voltagetransitions in the toggle flip flop circuit 122 are then applied tophase modulator 120 with a time delay of a similar magnitudeconstituting about half of one data bit length, so that voltage changesare invoked before passage of the rising edge of the data bit sequenceto be subjected to phase shifting. Since the signal on waveguide 136 isadvanced by half of the time period occupied by a data bit and thesignal on waveguide 138 is delayed by the same time period, the twocontrol signals are mutually displaced by a difference of one data bit.For example, the voltage transitions applied to phase modulator 120 canbe delayed relative to the voltage transitions applied to phasemodulator 118 by a combined time period within a range between about 75picoseconds and about 125 picoseconds, such as, for example, a timeperiod of about 100 picoseconds.

Accordingly, the same toggle flip flop circuit 122 can be used tocontrol both phase modulator 118 and phase modulator 120. When highvoltage is applied to phase modulator 120, data bar bit sequences remainin phase with the source signal. When low voltage is applied to phasemodulator 120, data bar bit sequences within the data bar bit sequencesignal are shifted to be about 180° out of phase with the source signal.

Phase modulator 118 emits an alternate block phase inverted data bitsignal on waveguide 142. The term “alternate block phase inversion” ismeant to convey that alternating data bit sequences are in an invertedphase. Similarly, phase modulator 120 emits an alternate block phaseinverted data bar bit signal on waveguide 144. The alternate block phaseinverted data bit signal emitted on waveguide 142 embodies a copy of thesource signal emitted by transmitter 102, in the form of data bitsequences. Mutually adjacent asserted data bits are not separated by anyreturn of the signal to an unasserted or zero state. For example, twoadjacent data bits are represented as a single composite pulse having atime length of two data bits. Mutually adjacent asserted data bitsequences, separated by an unasserted or zero data sequence having alength of at least one bit, are mutually out of phase by about 180°. Ifsuch mutually adjacent data bit sequences begin to overlap due tochromatic dispersion, destructive interference predominates theirinteraction. Therefore, a minimal amount of optical power is within thewidth of the unasserted data bit and the corruption of the original datasignal that is typically associated with the effects of chromaticdispersion on such mutually adjacent data bit sequences is significantlyreduced. The alternate block phase inverted data bar bit signal emittedon waveguide 144 is the inverse of the alternate block phase inverteddata bit signal emitted on waveguide 142. Hence, the alternate blockphase inverted data bar bit signal emitted on waveguide 144 effectivelyconstitutes an inverted copy of the alternate block phase inverted databit signal emitted on waveguide 142, providing protection against afailure in the delivery of either of the two signals.

The alternate block phase inverted data bit signal and the alternateblock phase inverted data bar bit signal are then directed throughoptical network 104 to receiver 106. The signals can be directed ondifferent paths through the optical network 104 in order to providefailure protection for the source signal. Optical network 104redundantly connects a plurality of nodes in the network via wavelengthpaths, either in a ring, mesh, or other optical network topology,whereby the alternate block phase inverted data bar bit signal and thealternate block phase inverted data bit signal may separately reach atermination node either directly or via multiple hops throughintermediate nodes.

The alternate block phase inverted data bit signal and alternate blockphase inverted data bar bit signal are then processed by receiver 106.For example, such signals may be converted from an optical form into anelectrical form by an optical to electrical converter. Since thealternate block phase inverted data bar bit signal is an invertedrepresentation of the source signal, it can be processed by the inverter114 to generate an alternate block phase inverted data bit signal.

In an alternative embodiment according to the present invention, thephase modulators 118 and 120 and toggle flip flop circuit 122 can beomitted. In such a case, waveguides 136 and 138 emit a non return tozero data bit sequence signal and a non return to zero data bar bitsequence signal, respectively. Since the data bar bit sequence signal isthe inverse of the data bit sequence signal, the data bar bit sequencesignal can be inverted to provide a copy of the data bit sequencesignal. Thus, failure protection is provided. However, no protectionagainst chromatic dispersion is afforded by this embodiment.

Background information regarding alternate block inversion coding of anon failure protected source signal is provided in the Laroia et al.U.S. Pat. No. 6,542,276, which is incorporated by reference herein inits entirety. Alternate block inversion coding of a source signal isreferred to by Laroia et al. as BAMI coding and is discussed inparticular at column 10, line 39 through column 11, line 33. Furtherbackground information regarding alternate block inversion coding of anon failure protected source signal is provided in Stark, Jason B. etal., “Line Coding for Dispersion Tolerance and Spectral Efficiency:Duobinary and Beyond”, Optical Fiber Communication Conference, 1999, andin the International Conference on Integrated Optics and Optical FiberCommunication, OFC/IOOC, Technical Digest, Vol. 2, pp. 351-333, 1999,both of which are incorporated herein by reference in their entirety.

FIG. 2 shows a binary representation of a portion of a suitable sourcedata stream 200 to be transmitted from transmitter 102 to receiver 106shown in FIG. 1. In one aspect according to the present invention,transmitter 102 utilizes source data stream 200 to generate an opticalsignal that is transmitted along waveguide 124. A series of exemplarybinary bits 205 within the source data stream 200 are shown, whichinclude asserted data bits such as exemplary asserted data bits 210 and215, and unasserted data bits such as exemplary unasserted data bits 220and 225. The series of binary bits 205 includes both solitary asserteddata bits such as exemplary asserted data bit 215, and mutually adjacentasserted data bits such as the three consecutive one bits collectivelyindicated at 230, for example. The series of binary bits 205 is suitablefor processing by the exemplary apparatus 100 according to the presentinvention as shown in FIG. 1, in order to provide the source data stream200 with resistance to chromatic dispersion and with protection againsta signal transmission failure. The source data stream 200 is transmittedby transmitter 102 on waveguide 124 to optical to electrical converter112. The resulting electrical signal is output on conductor 126 andinput into toggle flip flop circuit 122. Conductor 126 also carries theelectrical signal to conductor 128 which then inputs the electricalsignal into external modulator 116. Accordingly, the source data stream200 is used as a modulating control signal in external modulator 116 andtoggle flip flop circuit 122.

FIG. 3 shows a failure protected pair 300 of encoded signalrepresentations of the source data stream 200 shown in FIG. 2. Each ofthe asserted data bits in the source data stream 200 such as exemplaryasserted data bit 210, is represented within the encoded representation310 by electromagnetic radiation that is one bit length in duration.Each of the unasserted data bits in the source data stream 200 such asexemplary unasserted data bit 220, is represented within the encodedrepresentation 310 by an absence of electromagnetic radiation that isone bit length in duration such as exemplary unasserted data bit 316.Encoded signal representation 310 is a representation of the series ofbinary bits 205 within source data stream 200, and the asserted datathat the encoded representation 310 contains will be referred to as databits. Encoded signal representation 350 is an inverted representation ofthe series of binary bits 205 within source data stream 200, and theasserted data that the encoded representation 350 contains will bereferred to as data bar bits. FIG. 3 is shown together with FIG. 2 tomake these relationships visually apparent. The encoded representation310 includes data bit sequences comprising multiple asserted data bits,such as exemplary asserted data bit sequence 312. Data bit sequence 312represents the same data as do the three consecutive binary one bitscollectively indicated at 230 in FIG. 2. The three asserted data bits312 within data bit sequence 310 are mutually adjacent withoutinterposed unasserted data bits. The encoded representation 310 isaccordingly referred to as being in non return to zero format, becausethere is no return of the signal to zero in between the mutuallyadjacent asserted data bits in asserted data bit sequence 312. Encodedrepresentation 310 thus constitutes a non return to zero encoded databit sequence signal representing source data stream 200. For example,asserted data bits 314 and 318 in FIG. 3 represent asserted data bits210 and 215 in FIG. 2, respectively. Encoded representation 350 furtherconstitutes a non return to zero encoded data bar bit sequence signalrepresenting the inverse of the source data stream 200. For example,asserted data bar bits 352 and 354 in FIG. 3 represent unasserted databits 220 and 225 in FIG. 2, respectively. Inversion of the invertedencoded representation 350 yields a copy of encoded representation 310.Hence, encoded representations 310 and 350 can facilitate failureprotected transmission of source data stream 200, but do not provideprotection from chromatic dispersion.

Encoded representations 310 and 350 can be simultaneously generated inan efficient manner according to the present invention. Referring toFIG. 1, conductor 126 carries the electrical signal representing sourcesignal 205 to conductor 128 which then inputs the electrical signal intoexternal modulator 116. As explained earlier, external modulator 116generates constructive and destructive interference between the signalson waveguides 132 and 134, as modulated by the electrical signalrepresenting source signal 205. This constructive and destructiveinterference results in propagation of a series of asserted data bits onwaveguide 136 in the form of encoded signal representation 310. Externalmodulator 116 is adapted so that the phase of the interference appliedto the signal on waveguide 134 is inverted as compared with theinterference applied to the signal on waveguide 132. Accordingly, thesignal generated on waveguide 138 is the inverse of the signal generatedon waveguide 136, and is in the form of encoded signal representation350.

Encoded signal representation 310 is output on waveguide 136 shown inFIG. 1 and input to phase modulator 118. Encoded signal representation350 is output on waveguide 138 shown in FIG. 1 and input to phasemodulator 120. The electrical signal on conductor 126 is then used toenable the toggle flip flop circuit 122 to control the modulation ofphase modulators 118 and 120. As a result, alternating data bitsequences within the data bit sequence signal on waveguide 136 andalternating data bar bit sequences within the data bar bit sequencesignal on waveguide 138 are shifted 180° out of phase with each other.

FIG. 4 shows a failure protected pair 400 of encoded signalrepresentations 410 and 450 of source data stream 200 during signalmodulation by phase modulators 118 and 120 shown in FIG. 1. Encodedsignal representation 410 is the same signal representation as is databit signal representation 310 shown in FIG. 3. Encoded signalrepresentation 450 is the same signal representation as is data bar bitsignal representation 350 shown in FIG. 3. Time passes in the directionof the arrow t.

FIG. 4 further shows control signal waveform 430 aligned in time withencoded signal representation 410. Control signal waveform 430schematically indicates the points at which the phase of encoded signalrepresentation 410 is toggled between a normal position and an out ofphase position by application of the control signal from the toggle flipflop circuit 122. A relatively high voltage within control signalwaveform 430 is exemplified by the signal portion between control signalwaveform points 432 and 434. A relatively low voltage within controlsignal waveform 430 is exemplified by the signal portion between controlsignal waveform points 436 and 438. When a relatively high voltage isapplied to phase modulator 118, data bit sequences within data bitsequence signal 310 remain in phase with the source signal. When arelatively low voltage is applied to phase modulator 118, data bitsequences within data bit sequence signal 410 are shifted by a timeperiod equivalent to about half of the center wavelength of data bitsequence signal 410, or about 180°, out of phase with the source signal.Exemplary data bit sequence 412 has both a rising edge 413 at which atransition from a nonasserted bit to an asserted bit occurs, and asubsequent falling edge 414 at which a transition from an asserted bitto a nonasserted bit occurs. Following passage of the falling edge 414plus a time period of delay TD indicated at 415, control signal waveform430 registers a phase change at point 440. The time period of delay, TD,preferably constitutes about one half of the center wavelength of thecarrier signal. Accordingly, the time period of delay may be, forexample, about 50 picoseconds. Following passage of the next fallingedge 416 in data bit sequence 417 plus the same time period of delay TD,control signal waveform 430 registers another phase change at point 432.Phase changes 440 and 432 constitute a complete phase change cyclebrackets data bit sequence 417, that begins prior to arrival of therising edge 418 and ends after the arrival of falling edge 416. Hence,the portion of control signal waveform 430 spanning phase changes 440and 432 can be used to control the phase modulator 118 in order to shiftthe data bit sequence 417 out of phase.

Encoded representation 410 includes four additional falling edges 419,420, 421 and 422. Control signal waveform 430 includes the resultingneeded phase changes 441, 442, 444 and 446. Phase changes 441 and 442bracket and can be used to control phase shifting of data bit sequence423. Phase changes 444 and 446 bracket and can be used to control phaseshifting of data bit sequence 424. Asserted data bit sequences 412, 425and 426 will be referred to as regular data bit sequences. Theinterposed asserted data bit sequences 417, 423 and 424 will be referredto as alternate data bit sequences for the sole purpose of makingsubsequent reference to their interposed positions relative to theregular data bit sequences. Following completion of the phase shiftingas described, regular data bit sequences 412, 425 and 426 remain innormal phase, and alternate data bit sequences 417, 423 and 424 areshifted out of phase.

FIG. 4 also shows a time shifted control signal waveform 470 aligned intime with encoded signal representation 450. Control signal waveform 470is identical to control signal waveform 430, except that control signalwaveform 470 has been shifted forward in time by a time period ofadvancement TA indicated in FIG. 4 at 452. The time period ofadvancement TA, like the time period of delay TD, preferably constitutesabout one half of the center wavelength of the carrier signal.Accordingly, the time period of advancement may be, for example, about50 picoseconds.

Control signal waveform 470 can be used to control the phase shifting ofdata bar bit sequences 454, 456 and 458, while having no impact on databar bit sequences 460, 462 and 464. Asserted data bar bit sequences 460,462 and 464 will be referred to as regular data bar bit sequences. Theinterposed asserted data bar bit sequences 454, 456 and 458 will bereferred to as alternate data bar bit sequences for the sole purpose ofmaking subsequent reference to their interposed positions relative tothe regular data bar bit sequences. Following completion of the phaseshifting as described, regular data bar bit sequences 460, 462 and 464remain in normal phase, and alternate data bar bit sequences 454, 456and 458 are shifted out of phase.

The combined effect of an exemplary delay of control signal waveform 430by a time period of 50 picoseconds and an exemplary advancement ofcontrol signal waveform 470 by a time period of 50 picoseconds is toshift the two control signal waveforms in time relative to each other by100 picoseconds. Control signal waveform 430 can thus be used togenerate control signal waveform 470 simply by shifting control signalwaveform 430 relatively forward in time, compared with control signalwaveform 470, by 100 picoseconds. Such use of the output signal fromtoggle flip flop circuit 122 to generate both of the needed controlsignal waveforms provides for efficient completion of these signalprocessing operations. Alternatively, an inverted form of control signalwaveform 430 can be generated by the toggle flip flop circuit 122 andused as control signal waveform 470. The mutual symmetry embodied in thenon return to zero coding of representation 310 and its inverted copy,representation 350, facilitates this use of the same control signal toexecute the phase shifting of both representations 410 and 450 as shownin FIG. 4. Alternatively, control signal waveform 470 can beindependently generated in the same manner as described above withregard to control signal waveform 430.

FIG. 5 shows the resulting failure protected pair 500 of alternate blockphase inversion coded representations of the series of exemplary binarybits within source data stream 200. Alternate block phase inversioncoded representation 520 is the result of generation of encoded signalrepresentation 310 and processing of such encoded signal representationin the manner as shown in FIG. 4 by phase modulator 118. Alternate blockphase inversion coded representation 540 is the result of generating ofencoded signal representation 350 and processing of such encoded signalrepresentation in the manner as shown in FIG. 4 by phase modulator 120.

Data bit sequences 521, 522 and 523 within the encoded representation520 are aligned in time with and thus in phase with the source datastream 200. Data bit sequences 524, 525 and 526 have been shifted to beabout 180° out of phase with the source data stream 200 as representedby the crosshatching of these sequences. Hence, the electromagneticradiation waves within data bit sequences 524, 525 and 526 have beenshifted to a point that is behind the phase of the electromagneticradiation waves within data bit sequences 521, 522 and 523 by a timeperiod equivalent to about one half of the center wavelength of thecarrier signal. As the encoded representation 520 of the source datastream 200 is transmitted over a significant distance, the data bitsequences 521-526 spread out due to chromatic dispersion, as differentwavelengths within the source data stream 200 travel at slightlydifferent speeds. When the edges of adjacent asserted data bit sequencesbegin to overlap due to this pulse broadening, they destructivelyinterfere as a result of the alternate phase shifting. This destructiveinterference advantageously delays the onset of constructiveinterference. Constructive interference between adjacent data bitsequences can convert an intended unasserted data bit into an asserteddata bit, thus corrupting the source data stream 200. Hence, thealternate block phase inversion coding of encoded representation 520provides protection for the source data stream 200 against chromaticdispersion. So long as the overlap of adjacent data bit sequences is ofa magnitude less than about ⅓ of an asserted data bit, pulse broadeninginterference does not result in undue corruption of the original databit sequence signal. The alternate block phase shifting does not requirea magnitude of exactly 180°. Typically, the phase shifting can beselected within a range between about 135° and about 225°. Since perfectdestructive interference is achieved at 180°, however, a closeapproximation to 180° is desirable.

Encoded signal representation 540 is an inverted version of the encodedsignal representation 520. Asserted data bit sequences 521, 522, 523,524, 525 and 526 within encoded signal representation 520 representunasserted data bar bit sequences 541, 542, 543, 544, 545 and 546,respectively, within encoded signal representation 540. Unasserted databit sequences 527, 528, 529, 530, 531 and 532 within encoded signalrepresentation 520 represent asserted data bar bit sequences 547, 548,549, 550, 551 and 552, respectively, within encoded signalrepresentation 540. Conversion of the asserted and unasserted data barbit sequences of representation 540 into unasserted and asserted databit sequences, respectively, yields encoded signal representation 520.In a manner analogous to that described above with regard to encodedrepresentation 520, data bar bit sequences 548, 550 and 552 of encodedsignal representation 540 are in phase with the source data stream 200.However, as indicated by the crosshatching in FIG. 5, data bar bitsequences 547, 549 and 551 have been shifted to be about 180° out ofphase with the source data stream 200. Hence, the alternate block phaseinversion coding of encoded representation 540 also provides protectionfor the source data stream 200 against chromatic dispersion.

After completion of processing by phase modulator 118, alternate blockphase inversion coded representation 520 is then output on waveguide 142and input to optical network 104. Similarly, after completion ofprocessing by phase modulator 120, alternate block phase inversion codedrepresentation 540 is then output on waveguide 144 and input to opticalnetwork 104. Together, encoded representations 520 and 540 of the seriesof binary bits in exemplary source data stream 200 can facilitatefailure protected transmission of the source data stream 200 from anorigination point to a termination point. Encoded representations 520and 540 can be transmitted by different paths, so that transmissionfailure as to one of the representations can be remedied by receipt ofthe other representation at the termination point. For example, ifencoded representation 540 is needed to remedy a transmission failure asto representation 520, then representation 540 can be inverted byinverter 114 to constitute a copy of encoded representation 520.

FIGS. 6A and 6B together show an exemplary method 600 according to thepresent invention for producing alternate block phase inversion codedrepresentations 520 and 540 of a source data stream 200, which can ifdesired be carried out by using the exemplary apparatus 100 shown inFIG. 1. According to the method 600, first and second beams ofcontinuous wave electromagnetic radiation are provided at step 610, forexample using two coupled coherent continuous wave lasers. If desired, asingle continuous wave source of electromagnetic radiation such ascontinuous wave laser 108 can be provided at step 605, the continuouslaser wave of which is then suitably split using exemplary optical 1×2splitter 110 in order to yield the first and second beams needed at step610. At step 615, a series of binary bits is provided in the form ofsource data stream 200 by exemplary transmitter 102 to be transmittedfrom an origination point to a termination point.

In step 620, the first and second beams of electromagnetic radiation aremodulated with the source signal provided at step 615, in order togenerate first and second electromagnetic radiation signals at steps 630and 640, respectively. The first and second electromagnetic radiationsignals produced at steps 630 and 640 are shown in FIG. 3 as encodedsignal representations 310 and 350, respectively. Step 620 is carriedout in such a manner that whenever the source data stream 200 shown inFIG. 2 contains asserted data bits such as exemplary asserted data bit210, the first electromagnetic radiation signal shown as encodedrepresentation 310 contains asserted data bits such as exemplaryasserted data bit 314, and the second electromagnetic radiation signalshown as encoded representation 350 contains unasserted data bar bitssuch as exemplary unasserted data bar bit 356. Further, whenever thesource data stream 200 contains unasserted data bits such as exemplaryunasserted data bit 220, the first electromagnetic radiation signalshown as encoded representation 310 contains unasserted data bits suchas exemplary unasserted data bit 316, and the second electromagneticradiation signal shown as encoded representation 350 contains asserteddata bar bits such as exemplary asserted data bar bit 352.

Encoded representation 350 constitutes the inverse of encodedrepresentation 310. Therefore, step 620 can be considered as requiringthe source data stream 200 to at all times be represented either byasserted data bits in encoded representation 310 or by asserted data barbits in encoded representation 350, but never simultaneously by both.Hence, step 620 calls for switching a continuous wave between encodedrepresentation 310 and inverted encoded representation 350 as modulatedby the source data stream 200. Step 620 efficiently generates invertedencoded representation 350, since inverted encoded representation 350 isthe inverse of encoded representation 310. The electromagnetic radiationthat would be discarded if only encoded representation 310 were beinggenerated, is instead employed to generate inverted encodedrepresentation 350. The non return to zero coding in representations 310and 350 is particularly suitable for generation by step 620, as thiscoding format permits direct processing of an inverted copy of the codedsignal by the switching process, without any further signal conversionor other processing being required. Step 620 can be carried out by anyhardware or software suitable to carry out this switching process. Ingeneral, direct modulation of the source data stream 200 leads toperturbations in the leading and falling edges of data sequences that isreferred to as “chirp”. Indirect, external modulation of the source datastream 200 can be carried out to minimize such perturbations.

If desired, step 620 can be carried out as specified at step 625, bymodulating the relative phases of the first and second beams with thesource signal and then directing the two resulting output signalsthrough a propagation medium to facilitate their mutual interference.Step 620 can thus be carried out by using the exemplary externalmodulator 116. Optical to electrical converter 112 provides the neededcontrol signal, modulated by the source data stream 200. Constructiveand destructive interference between the first and second beams can thusbe modulated by the source signal so that asserted data bits aregenerated within encoded representation 310 and asserted data bar bitsare generated within encoded representation 350.

Steps 620 and 625 result in generation at step 630 of encodedrepresentation 310 constituting a first electromagnetic radiationsignal. Step 630 as shown in FIG. 6 summarizes features of encodedrepresentation 310. Encoded representation 310 is constituted by binarydata including asserted data bits and unasserted data bits. The asserteddata bits are coded in non return to zero format. Mutually adjacentasserted data bits are conjoined as exemplified by asserted data bitsequence 312. The term “conjoined” as used herein means that themutually adjacent asserted data bits are directly joined together into aunitary asserted data bit sequence having a length equivalent to thecumulative data bit lengths that are so joined together. Encodedrepresentation 310 is considered at this stage to include regularasserted data bit sequences interposed by alternate asserted data bitsequences, for the sole purpose of facilitating subsequent reference totheir interposed positions relative to the regular asserted data bitsequences. Encoded representation 310 is output by exemplary externalmodulator 116 on waveguide 136 and input to phase modulator 118.

Steps 620 and 625 also result in generation at step 640 of encodedrepresentation 350 constituting a second electromagnetic radiationsignal. Step 640 as shown in FIG. 6 summarizes features of encodedrepresentation 350. Encoded representation 350 is constituted by binarydata including asserted data bar bits and unasserted data bar bits. Theasserted data bar bits are coded in non return to zero format. Mutuallyadjacent asserted data bar bits are conjoined as exemplified by asserteddata bar bit sequence 358. The asserted data bar bits represent theunasserted data bits. The unasserted data bar bits represent theasserted data bits. Encoded representation 350 is considered at thisstage to include regular asserted data bar bit sequences interposed byalternate asserted data bar bit sequences, for the sole purpose offacilitating subsequent reference to their interposed positions relativeto the regular asserted data bar bit sequences. Encoded representation350 is output by exemplary external modulator 116 on waveguide 138 andinput to phase modulator 120.

If desired, in steps 650 and 655 the first and second electromagneticradiation signals, respectively, can be modulated with the source signalto shift the phase of alternate data bit and data bar bit sequences.These steps can be carried out in the manner as discussed in detailabove in connection with FIG. 4. If desired, phase modulators 118 and120, controlled by toggle flip flop circuit 122 itself modulated by thesource data stream 200, can be used to carry out these steps. Step 650yields a data bit sequence signal in the format of representation 520.Step 655 yields a data bar bit sequence signal in the format ofrepresentation 540.

At step 660, the data bit sequence signal in the format ofrepresentation 520 and the data bar bit sequence signal in the format ofrepresentation 540 are transmitted over different paths from anorigination point to a termination point. For example, representations520 and 540 can be input by waveguides 142 and 144 onto diverse routesthrough exemplary optical network 104. The alternate block phaseinversion coding provides each of the two signals with resistance tochromatic dispersion. The duality of the signals as representing thesame information and transmitted over different paths providesprotection against the failure of delivery of either one of the signalsto the termination point.

At step 665, the data bar bit sequence signal in the format ofrepresentation 540 can be decoded by inversion if desired, to generate adata bit sequence signal in the format of representation 520. This stepcan be carried out by using exemplary inverter 114 shown in FIG. 1.

FIG. 7 shows another exemplary embodiment of a suitable apparatus 700that can be used to produce a chromatic dispersion resistantelectromagnetic radiation signal protected against a signal transmissionfailure in accordance with the present invention.

The apparatus 700 includes a transmitter 702 from which a source signaloriginates and which needs to be transmitted to a receiver 704. Theapparatus further includes a continuous wave laser 706, an optical toelectrical converter 708, an integrated external modulator with phasemodulators 710, a flip flop circuit 712, an optical network 714, and aninverter 716. The integrated external modulator with phase modulators710 comprises an optical 1×2 splitter 718. The integrated externalmodulator with phase modulators 710 further comprises electrodes 720,722, 724, 726, 728, 730, 787, 788, 789, 790, 791 and 792 located on adielectric substrate 732. Electrodes 720, 722, 787 and 788 together forman external modulator. Waveguide 746 longitudinally passes nearby andbetween electrodes 720 and 787. Waveguide 748 longitudinally passesnearby and between electrodes 722 and 788. Waveguides 746 and 748 thenpass in mutual proximity to form a coupler 750, and then diverge intowaveguides 752 and 754, respectively. Electrodes 724, 726, 789 and 790together form a first phase modulator generally indicated at 756, andelectrodes 728, 730, 791 and 792 together form a second phase modulatorgenerally indicated at 758. Waveguide 752 branches into two separatewaveguides 760 and 762. Waveguide 760 longitudinally passes betweennearby electrodes 724 and 789. Waveguide 762 longitudinally passesbetween nearby electrodes 726 and 790. Waveguides 760 and 762 thenrejoin to form waveguide 764, forming phase modulator 756. Waveguide 754branches into two separate waveguides 766 and 768. Waveguide 766longitudinally passes between nearby electrodes 728 and 791. Waveguide768 longitudinally passes between nearby electrodes 730 and 792.Waveguides 766 and 768 then rejoin to form waveguide 770, forming phasemodulator 758.

The transmitter 702 transmits an optical source signal on waveguide 772to optical to electrical converter 708. Optical to electrical converter708 outputs an electrical signal on conductor 774 which is input intotoggle flip flop circuit 712. Optical to electrical converter 708 alsooutputs an electrical signal on conductor 776 which is input intoelectrodes 720 and 722 in the integrated external modulator with phasemodulators 710. Electrodes 787 and 788 are suitably grounded byconductor 775. Continuous wave laser 706 outputs a continuous opticalwave on waveguide 778 to optical 1×2 splitter 718, which is included inthe integrated external modulator with phase modulators 710. Optical 1×2splitter 718 outputs continuous optical waves on waveguides 746 and 748.The optical wave on waveguide 746 is conveyed longitudinally betweenelectrodes 720 and 787, through coupler 750, and then on waveguide 752to phase modulator 756. The optical wave on waveguide 748 is conveyedlongitudinally between electrodes 722 and 788, through coupler 750, andthereafter on waveguide 754 to phase modulator 758. The optical signalon waveguide 752 is then split into two identical signals on waveguides760 and 762. Waveguide 760 carries the optical signal between nearbyelectrodes 724 and 789. Waveguide 762 carries the optical signal betweennearby electrodes 726 and 790. Waveguides 760 and 762 then recombine theoptical signals at waveguide 764, forming phase modulator 756. Theoptical signal is then output into optical network 714. The opticalsignal on waveguide 754 is split into two identical signals onwaveguides 766 and 768. Waveguide 766 carries the optical signallongitudinally between nearby electrodes 728 and 791. Waveguide 768carries the optical signal longitudinally between nearby electrodes 730and 792. Waveguides 766 and 768 then recombine the optical signals atwaveguide 770, forming phase modulator 758. The optical signal is thenoutput into optical network 714. Toggle flip flop circuit 712 outputs anelectrical signal on conductor 780 which is input to electrodes 724,726, 728 and 730. Electrodes 789, 790, 791 and 792 are suitably groundedby conductor 779. After passing through the optical network 714, theoptical signal received from waveguide 764 is input on waveguide 782 toinverter 716, and then on waveguide 784 to receiver 704. After passingthrough the optical network 714, the optical signal received fromwaveguide 770 is input on waveguide 786 to receiver 704.

In operation of the apparatus 700, continuous wave laser 706 executesstep 605 by generating a continuous wave of electromagnetic radiation onwaveguide 778 which is directed into integrated external modulator withphase modulators 710. The output power of the continuous wave laser 706is adjusted to provide electromagnetic radiation of adequate intensityto produce two continuous electromagnetic radiation waves each having anappropriate intensity. The 1×2 splitter generally indicated at 718within the integrated external modulator with phase modulators 710 thenproduces two identical continuous waves of electromagnetic radiation onwaveguides 746 and 748, executing step 610.

Integrated external modulator with phase modulators 710 is provided withelectrodes 720 and 787 positioned as discussed above with respect towaveguide 746, and with electrodes 722 and 788 positioned as discussedabove with respect to waveguide 748, constituting a 1×2 dual outputMach-Zehnder interferometer. In the exemplary embodiment an x-cut designis used, as particularly useful for signals carried on anelectromagnetic radiation wave having a bit rate of 10 Gb/sec or less.Alternatively, a z-cut design can be used, as particularly useful forsignals carried on an electromagnetic radiation wave having a bit rateof at least 10 gigabytes per second (Gb/sec). Further alternativeexternal modulator designs can be substituted.

A source signal is emitted from the transmitter 702 on waveguide 772 anddirected into optical to electrical converter 708. The optical toelectrical converter 708 converts the source signal on waveguide 772from an optical form into an electrical form. The resulting electricalsignal is then emitted from the optical to electrical converter 708 onconductor 774 and is directed by conductor 776 to electrodes 720 and722, executing step 615.

The continuous waves of electromagnetic radiation on waveguides 746 and748 are directed past electrodes 720 and 722 carrying an electricalsignal modulated by the source signal. Electrodes 720 and 722accordingly emit electromagnetic fields modulated by the source signalemitted by transmitter 702, which modulate the phase of the waves ofelectromagnetic radiation on waveguides 746 and 748. Since electrodes720 and 722 are on opposite sides of the waveguides from groundedelectrodes 787 and 788, the field effect of the source signal on thephases of the waves of electromagnetic radiation on waveguides 746 and748 is of opposite polarities. Consequently, the phases of the waves ofelectromagnetic radiation on waveguides 746 and 748 are simultaneouslychanged in opposite directions. The relative phases of the waves ofelectromagnetic radiation on waveguides 746 and 748 can be modulatedbetween an applied voltage (V) of Vπ/2 and −Vπ/2 in order to generateconstructive and destructive interference between the waves ofelectromagnetic radiation, thus executing step 620 and initiating step625.

Waveguides 746 and 748 then direct the waves of electromagneticradiation as two separate output signals into close mutual proximitythrough the coupler 750, where they constructively and destructivelyinterfere as modulated by the source signal. The coupler is an effectivepropagation medium to facilitate mutual interference of the waves ofelectromagnetic radiation in completion of step 625. This interferenceresults in emission of a data bit sequence signal 310 in execution ofstep 630 which is then directed by waveguide 752 into phase modulator756, and emission of a data bar bit sequence signal 350 in execution ofstep 640 which is then directed by waveguide 754 into phase modulator758.

The electrical signal on conductor 774 is also used to control phasemodulators 756 and 758. The electrical signal is directed into toggleflip flop circuit 712, which senses transitions in the source signalfrom asserted data bits to unasserted data bits and operates in the samemanner as previously discussed in connection with FIG. 4. The toggleflip flop circuit 712 can be provided, for example, by a Fraunhofer IAFASD201M static divider. The voltage emitted by toggle flip flop circuit712 is directed by conductor 780 to control phase modulator 756 andphase modulator 758. The signal emitted by toggle flip flop circuit 712is advanced as directed to phase modulator 758 and delayed as directedto phase modulator 756 in the same manner as discussed above inconnection with FIG. 4. The same toggle flip flop circuit 712 can beused to control both phase modulator 756 and phase modulator 758.Alternatively, if desired, separate toggle flip flop circuits can beprovided to control each of the phase modulators 756 and 758.

Phase modulator 756 is provided with electrodes 724 and 726, and phasemodulator 758 is provided with electrodes 728 and 730. Electrodes 724,726, 728 and 730 are supplied with a control signal generated by toggleflip flop circuit 712 and carried by conductor 780. Electrodes 724, 726,728 and 730 accordingly emit electromagnetic fields modulated by thesource signal emitted by transmitter 702.

When a relatively high voltage as discussed above in connection withFIG. 4 is applied to electrodes 728 and 730 of phase modulator 758, databit sequences within data bit sequence signal 310 remain in phase withsource signal. When a relatively low voltage is applied to suchelectrodes of phase modulator 758, data bit sequences within data bitsequence signal 310 are shifted about 180° out of phase with the sourcesignal. When a relatively high voltage as discussed above in connectionwith FIG. 4 is applied to electrodes 724 and 726 of phase modulator 756,data bar bit sequences within data bar bit sequence signal 350 remain inphase with source signal. When a relatively low voltage is applied tosuch electrodes of phase modulator 756, data bar bit sequences withindata bar bit sequence signal 350 are shifted about 180° out of phasewith the source signal.

Phase modulator 758 emits an alternate block phase inverted data bitsignal on waveguide 770 in execution of step 650 in the form of encodedsignal representation 520 shown in FIG. 5. Phase modulator 756 emits analternate block phase inverted data bar bit signal on waveguide 764 inexecution of step 655 in the form of encoded signal representation 540shown in FIG. 5.

The alternate block phase inverted data bit signal and the alternateblock phase inverted data bar bit signal are then directed throughoptical network 714 to receiver 704, in execution of step 660. Thesignals can be directed on different paths through the optical network714 in order to provide failure protection for the source signal.

The alternate block phase inverted data bit signal in the form ofencoded signal representation 520 and alternate block phase inverteddata bar bit signal in the form of encoded signal representation 540 areconventionally processed by receiver 704. For example, signals 520 and540 may be converted from an optical form into an electrical form by anoptical to electrical converter. Since encoded signal representation 540is in an inverted format, it can be processed by inverter 716 inexecution of step 665.

In an alternative embodiment according to the present invention, thephase modulators 756 and 758 and toggle flip flop circuit 712 can beomitted. In such a case, waveguides 754 and 752 emit a non return tozero data bit sequence signal and a non return to zero data bar bitsequence signal, respectively. These signals are then transmittedthrough the network 714. In a further alternative embodiment accordingto the present invention, electrodes 787, 788, 789, 790, 791 and 792 cancarry an electrical signal modulated by the source signal, andelectrodes 720, 722, 724, 726, 728 and 730 can be suitably grounded.

The apparatus shown in FIGS. 1 and 7 and discussed above are nonlimiting exemplary embodiments suitable for use in accordance with thepresent invention. FIGS. 1 and 7, for example, each show a singletransmitter and a single receiver. Telecommunications networks typicallyinclude a plurality of transmitters and receivers, facilitating two waysignal transmission between any two network locations. The methods andapparatus according to the present invention have been discussed inconnection with providing one inverted copy of a source signal asprotection against one signal transmission failure. However, if desired,further direct and inverted copies of a source signal could also beprovided in the same manner as described.

Other designs for external modulators and phase modulators capable ofprocessing electromagnetic radiation waves to generate modulated phasechanges and modulated constructive and destructive interference can alsobe used. Suitable alternative devices may include devices from theclasses known as electro absorption and electro optic modulators, theformer class including devices composed of materials used insemiconductor lasers, and the latter class comprising materials whoserefractive index can be altered by an applied electric field. Internalmodulators not subject to signal distortions such as chirping can ifavailable be used. The co pending and co owned Korotky et al. U.S.patent application Ser. No. 10/245,029, filed on Sep. 17, 2002, entitled“Provisionable Keep-Alive Signal For Physical-Layer Protection of anOptical Network” (Korotky et al.) discloses external modulators havingintegral splitters, that are suitable for use in accordance with thepresent invention. Phase modulators as shown in FIGS. 1 and 7 herein canbe added to such external modulators. The entirety of the Korotky et al.application, with particular attention to FIGS. 4 and 7 and theaccompanying discussion, is incorporated by reference herein in itsentirety. Further background information regarding Mach-Zehnder devicesis provided in Wooten, Ed L. et al., “A Review of Lithium NiobateModulators for Fiber-Optic Communications Systems,” IEEE Journal ofSelected Topics in Quantum Electronics, Vol. 6, No. 1, pp. 69-82,January/February 2000, which is hereby incorporated by reference hereinin its entirety. The present invention may be implemented usingMach-Zehnder modulators of the lithium niobate type, although othersuitable types of Mach-Zehnder modulators and suitable modulators otherthan Mach-Zehnder modulators may also be used.

In order to demonstrate the quality characteristics of signals with andwithout phase inversion prepared according to the present invention, atest apparatus was constructed. The test apparatus was consistent withFIG. 1 as described above, except that phase modulator 120, inverter114, and network 104 were omitted.

FIG. 8 shows a graph 800 plotting on the vertical axis, bit error rate(BER), and plotting on the horizontal axis the resulting optical signalto noise ratio (OSNR) required in decibels (dB) in order to transmit thesignal over a distance of 80 kilometers (km) on standard single modeoptical fiber (SSMF). Accordingly, FIG. 8 represents the impact ofvarying the BER while holding the transmission distance constant. Thebit error rate is a ratio of bit errors as a fraction of total bits andis a measure of signal quality. In the experiments a (2³¹−1) bitpseudorandom bit sequence generator was employed. Curve 810 plotsmeasured data for a signal without phase shifting. Curve 820 plotsmeasured data for an alternate block phase inverted signal, in whichalternate data bit sequences were shifted about 180° out of phase. Curve810 shows that the experimental apparatus without alternate block phaseshifting effectively coded the signal, with an OSNR of about 16.4 dBrequired to achieve a BER of about 1×10⁻³. Curve 820 shows that theexperimental apparatus with alternate block phase shifting alsoeffectively coded the alternate block phase inversion signal, with anOSNR of only about 14 dB required to achieve a BER of about 1×10⁻³.Since 3 dB approximately represents a doubling in signal strength, thedisparity between curves 810 and 820 confirms the substantialimprovement in signal quality resulting from generation of the alternateblock phase inversion signal format according to the present invention.

FIG. 9 shows a graph 900 that plots the OSNR in dB on the vertical axis,and the distance traveled in km by the signal over SSMF on thehorizontal axis, with a fixed BER of 1×10⁻³. Accordingly, FIG. 9represents the impact on OSNR of varying the transmission distance whileholding the BER constant. Curve 910 again plots measured data for asignal without phase shifting. Curve 920 plots measured data for analternate block phase inverted signal, in which alternate data bitsequences were shifted about 180° out of phase. At a transmissiondistance of about 80 km, curves 910 and 920 indicate a required OSNR ofabout 16 dB and about 13.5 dB, respectively. Both of these OSNR's areacceptable, but the performance of the alternate block phase invertedsignal is clearly superior.

In the above embodiments, the laser, external modulator, toggle flipflop circuit, and other apparatus elements have been depicted inseparate boxes. In FIG. 7, the external modulator and phase modulatorsare integrated together. Depending on the implementation, differentparts of the components for use in accordance with the present inventionmay be implemented in the same or different housings, circuit packs,circuit cards, multi-chip modules, substrates, or mixed-mode applicationspecific integrated circuits, potentially along with other circuitry. Inone possible implementation, the laser and the external modulator areintegrated together onto the same substrate.

The present invention has been described broadly with respect tonetworks operated using electromagnetic radiation such as light at acenter wavelength of, for example, about 1550 nm. However, it should beunderstood by one skilled in the art that the present invention isequally applicable to related systems, subsystems and apparatus,including synchronous optical networks (SONET), add-drop multiplexers,and optical internet-protocol (IP) routers.

The present invention has further been described with respect toprotection of a source signal. The apparatus and methods according tothe present invention are also applicable to bridging applications inwhich a plurality of representations of a source signal are transmittedto a plurality of destinations.

While the present invention has been disclosed in the context of variousaspects of presently preferred embodiments, it will be recognized thatthe invention may be suitably applied to other environments consistentwith the claims which follow.

1. An apparatus for creating alternate block phase inversion codedrepresentations of an optical communication signal, the apparatuscomprising: a modulator adapted to modulate a first and a second beam ofcontinuous wave electromagnetic radiation with a source signal so thatthe first and second beams combine to produce streams of asserted andunasserted data bits, to assemble modulated portions of said first andsecond beams into a first electromagnetic radiation signal of interposedregular and alternate data bit sequences comprising blocks of assertednon return to zero coded data bits, each of said data bit sequencesbeing interposed by blocks of unasserted data bits, and to assemblemodulated portions of said first and second beams into a secondelectromagnetic radiation signal of interposed regular and alternatedata bar bit sequences comprising blocks of asserted non return to zerocoded data bar bits representing said unasserted data bits, each of saiddata bar bit sequences being interposed by blocks of unasserted data barbits representing said asserted data bits; and means for modulating saidfirst and second electromagnetic radiation signal using informationprovided by said source signal, to shift the phase of said alternatedata bit sequences and said alternate data bar bit sequences.
 2. Theapparatus of claim 1 wherein the means for modulating said first andsecond electromagnetic radiation signals employs a control signal timeshifted for at least one of the first and second electromagneticradiation signals.
 3. The apparatus of claim 1, in which said modulatoris an external modulator that is adapted to modulate the relative phasesof said first and second beams of continuous wave electromagneticradiation with said source signal and to then subject said first andsecond beams of electromagnetic radiation to mutual interference.
 4. Theapparatus of claim 1 comprising means adapted to decode said secondelectromagnetic radiation signal into a copy of said firstelectromagnetic radiation signal, by converting said unasserted data barbits into asserted data bits and by converting said asserted data barbits into unasserted data bits.
 5. The apparatus of claim 1 in whichsaid modulator is adapted to modulate first and second beams ofcontinuous wave light.
 6. The apparatus of claim 1 further comprisingmeans for transmitting said first and second electromagnetic radiationsignals to a single destination, providing protection for said sourcesignal against a signal transmission failure.
 7. The apparatus of claim1 further comprising means for transmitting said first and secondelectromagnetic radiation signals to two destinations, bridging saidsource signal to said two destinations.
 8. The apparatus of claim 1further comprising: a transmitter for providing said source signal; areceiver; and an optical network having a first path and a second path,each of said paths being in communication with said transmitter and saidreceiver; said apparatus adapted to transmit said first electromagneticradiation signal from said transmitter to said receiver on said firstpath and to transmit said second electromagnetic radiation signal fromsaid transmitter to said receiver on said second path.
 9. The apparatusof claim 1 further comprising: a transmitter for providing said sourcesignal; first and second receivers; and an optical network having afirst path and a second path, said first path being in communicationwith said transmitter and said first receiver, and said second pathbeing in communication with said transmitter and said second receiver;said apparatus adapted to transmit said first electromagnetic radiationsignal from said transmitter to said first receiver on said first pathand to transmit said second electromagnetic radiation signal from saidtransmitter to said second receiver on said second path.
 10. Theapparatus of claim 1 in which said means for modulating said first andsecond electromagnetic radiation signals comprises first and secondphase modulators.
 11. The apparatus of claim 10 in which the means formodulating is adapted to simultaneously shift the phases of saidalternate data bit sequences and of said alternate data bar bitsequences by directing modulation of said first and secondelectromagnetic radiation signals using the information provided by saidsource signal.
 12. The apparatus of claim 3 in which said externalmodulator comprises a dual output intensity modulator.
 13. The apparatusof claim 11 in which said means adapted to simultaneously shift thephases of said alternate data bit sequences and of said alternate databar bit sequences comprises a toggle flip flop circuit.
 14. A method ofcreating alternate block phase inversion coded representations of anoptical communication signal, comprising: modulating a first and asecond beam of continuous wave electromagnetic radiation with a sourcesignal so that the first and second beams combine to generate streams ofasserted and unasserted data non return to zero coded bits; generating afirst electromagnetic radiation signal of interposed regular andalternate data bit sequences comprising blocks of the asserted nonreturn to zero coded data bits, each of said data bit sequences beinginterposed by blocks of the unasserted data bits; generating a secondelectromagnetic radiation signal of interposed regular and alternatedata bar bit sequences comprising blocks of asserted non return to zerocoded data bar bits representing said unasserted data bits, each of saiddata bar bit sequences being interposed by blocks of unasserted data barbits representing said asserted data bits; modulating said firstelectromagnetic radiation signal using information provided by saidsource signal to shift the phase of said alternate data bit sequences;and modulating said second electromagnetic radiation signal usinginformation provided by said source signal to shift the phase of saidalternate data bar bit sequences.
 15. The method of claim 14 furthercomprising: utilizing a common control signal time shifted for at leastone of the modulating steps in said modulating said first and secondelectromagnetic radiation signal.
 16. The method of claim 14 in whichsaid step of modulating comprises modulating the relative phases of saidfirst and second beams of continuous wave electromagnetic radiation withsaid source signal to produce first and second output signals, and thenlaunching said first and second output signals into a propagation mediumsuch that said first and second output signals mutually interfere,producing said first and second electromagnetic radiation signals. 17.The method of claim 14 comprising the further step of decoding saidsecond electromagnetic radiation signal into a copy of said firstelectromagnetic radiation signal by converting said unasserted data barbits into asserted data bits and by converting said asserted data barbits into unasserted data bits.
 18. The method of claim 14 comprisingthe further step of transmitting said first electromagnetic radiationsignal and said second electromagnetic radiation signal from anorigination point to a termination point over two different paths toprovide protection for said source signal against a signal transmissionfailure.
 19. The method of claim 14 comprising the further step oftransmitting said first electromagnetic radiation signal and said secondelectromagnetic radiation signal from an origination point to twodifferent termination points over two different paths to providebridging of said source signal.
 20. The method of claim 14 in which eachof said first electromagnetic radiation signal and said secondelectromagnetic radiation signal is an optical signal.
 21. The method ofclaim 14 in which said steps of modulating said first and secondelectromagnetic radiation signal with said information provided by saidsource signal comprise simultaneously shifting the phases of saidalternate data bit sequences and said alternate data bar bit sequences.22. The method of claim 14 comprising the further step of transmittingsaid first and second electromagnetic radiation signals over a distancesufficient to generate chromatic dispersion resulting in some overlapbetween said data bit sequences and in some overlap between said databar bit sequences, producing destructive interference.
 23. The method ofclaim 14 in which said step of modulating said first and secondelectromagnetic radiation signals with said source signal comprisesshifting said phases by about 180°.
 24. The method of claim 16 in whichsaid step of modulating comprises controlling such mutual interferenceto selectively and simultaneously create said asserted data bits andsaid unasserted data bar bits.